A compressed air control apparatus is known from German Patent No. DE 195 44 621 C1 corresponding to European Patent No. EP 0 776 807 B1. A pressure controller and an air dryer including respective elements are located in a common housing. The common housing may also be designed to be divided. The known compressed air control apparatus includes a multi circuit protection valve being designed and arranged to serve for a plurality of circuits being connected to the housing. Each circuit includes an overflow valve having limited backflow. For this purpose, each overflow valve includes a valve body or a so called “movable wall” being supported on a spring at one side. An effective surface is located at the other side. The effective surface is subjected or impinged by a compressed air in a flow chamber being located at the upstream side. The flow chamber is permanently connected to a central bore serving for distribution of the arriving compressed air. The valve body of the overflow valve includes two effective surfaces facing the upstream side, namely the aforementioned first effective surface and a second effective surface. The second effective surface is active in the opened position, and it is subjected with compressed air in the opened position, respectively. The valve body is supported on a spring being surrounded by a spring chamber. A bypass conduit is located between the flow chamber and the spring chamber. A solenoid valve including its own deaerating connection is located in the bypass conduit. The solenoid valve is closed in its currentless condition such that the spring chamber of the overflow valve is connected to the atmosphere. When the solenoid valve is actively controlled, it switches to reach its other position. This means that the deaerating connection is locked and the spring chamber is connected to the flow chamber such that there is an additional force or auxiliary force in addition to the force supplied by the spring. The auxiliary force tends to close the overflow valve and to keep the overflow valve closed, respectively. In this way, the overflow valve fulfills a plurality of functions. When the rear chamber is connected to the atmosphere, it functions in the common way, meaning it provides the advantageous functions of an overflow valve having limited backflow. When the rear chamber is supplied with the same pressure as the control chamber at the other side of the valve body, the force of the spring in the rear chamber closes the overflow valve. This means that it also fulfills the function of the locking valve. In this way, it is possible to lock the respective circuit, especially in case of a failure. This has a positive effect on the other unlocked circuits, meaning emergency operation may also be maintained for these circuits during a failure. Furthermore, the multi circuit protection valve and the respective overflow valve may also be used for the purpose of limiting the pressure. It is a drawback that the known overflow valve having limited backflow cannot be opened before having reached the opening pressure. Another essential drawback of the known compressed air control apparatus is the fact that there is increased pressure in this circuit having limited pressure during failure of the electrical supply or when ignition of the vehicle is turned off when using different pressures in the circuits and using an overflow valve as the pressure limiting device. This increased pressure is forbidden by law.
Another similar compressed air control apparatus is known from German Patent No. DE 197 00 243 C1. The effective surface to be directly subjected by the solenoid valve is located at the side of the valve body facing away from the flow chamber and, thus, at the side of the spring. A special pressure limiting valve is used for the following circuits. The pressure limiting valve is arranged downstream of the respective overflow valve of the respective circuit. In this case, the overflow valve is only used to fulfill its normal pneumatic function. The pressure limiting valve may be additionally influenced by the solenoid valve. However, it is not possible to open the overflow valve below the predetermined opening pressure in an arbitrary way, meaning when it is desired. The actuation of the solenoid valve results in the pressure limiting function of the pressure limiting device being turned off.
Another compressed air control apparatus is known from German Patent No. DE 44 21 575 C2 corresponding to European Patent No. EP 0 689 117 B1. The known apparatus includes a pressure controller, an air dryer and a multi circuit protection valve. The circuits are protected by check valves being pushed open by an actuation piston to be switched by a solenoid valve. The circuits may also be protected by an overflow valve including limited backflow. A solenoid valve is associated with each of the overflow valves of each circuit. The solenoid valve in its non-excited condition is opened, meaning the spring chamber of the overflow valve is connected to the flow chamber. In this way, the overflow valve has a comparatively increased opening pressure. In the excited position, the solenoid valve switches, and it deaerates the spring chamber. The effective surface to be subjected by the solenoid valve is located at the side of the movable wall facing away from the flow chamber, meaning in the spring chamber. This results in the insofar increased effective surface of the spring chamber having the effect of the overflow valve not opening when filling. This is only possible when the solenoid valve switches to reach the position in which the spring chamber is connected to the atmosphere. In this way, there is the drawback of supply of the circuits not being possible during a failure of the electric supply. On the other hand, when the electric supply works correctly, it is not possible to open the overflow valve of a circuit before having reached the predetermined opening pressure of the overflow valve.